Parent Company Financial Information - STATEMENTS OF OPERATIONS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income
Interest income	$ 85,684	$ 70,161	$ 68,733	$ 45,427	$ 45,797	$ 42,429	$ 40,874	$ 37,505	$ 270,005	$ 166,605	$ 118,356
Noninterest income	9,451	8,221	8,759	4,683	4,318	5,968	4,468	4,848	31,114	19,602	14,388
Expense
Interest expense	7,514	5,870	5,395	3,724	3,493	3,420	3,313	3,304	22,503	13,530	12,057
Noninterest expense	49,895	39,612	48,496	29,747	25,377	25,860	23,713	23,633	167,750	98,583	73,538
Net income before income taxes									102,226	65,318	40,724
Income tax	19,370	10,619	7,521	4,616	7,238	5,877	6,358	5,742	42,126	25,215	15,209
Net income	$ 16,171	$ 20,232	$ 14,176	$ 9,521	$ 11,953	$ 9,227	$ 10,369	$ 8,554	60,100	40,103	25,515
Corporation
Income
Interest income									36	31	27
Noninterest income									0	0	0
Total income									36	31	27
Expense
Interest expense									4,720	3,844	3,937
Noninterest expense									8,956	3,769	2,831
Total expense									13,676	7,613	6,768
Net income before income taxes									(13,640)	(7,582)	(6,741)
Income tax									(5,417)	(2,785)	(2,783)
Net loss (parent only)									(8,223)	(4,797)	(3,958)
Equity in net earnings of subsidiaries									68,323	44,900	29,473
Net income									$ 60,100	$ 40,103	$ 25,515